Odd Alien Planets So Close Together They See 'Planetrise'

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It is science fiction made fact: Astronomers have discovered two
alien planets around the same star whose orbits come so close
together that each rises in the night sky of its sister world
like an exotic full moon.

The newfound planets are 1,200 light-years from Earth and an
unprecedented find, researchers said. They differ greatly in size
and composition but come within just 1.2 million miles (1.9
million kilometers) of each other, closer than any other pair of
planets known, according to a new study.

One of the newly discovered
alien planets, called Kepler-36b, appears to be a rocky
"super-Earth" 4.5 times as massive as our planet. The other,
Kepler-36c, is a gaseous, Neptune-size world about eight times as
massive as Earth. The two planets meet up every 97 days in a
conjunction that would make each dramatically visible in the
other's sky.

“These two worlds are having close encounters,” said co-lead
author Josh Carter, of the Harvard-Smithsonian Center for
Astrophysics, in a statement.

At their closest approach, the two planets are separated by five
times the distance between the Earth and the moon. How such
different bodies ended up in such similar orbits may be tough for
current theories of
planet formation and migration to explain, researchers said.

"This is unprecedented," co-lead author Eric Agol, of the
University of Washington, told SPACE.com via email. "They are as
different in density as Earth and Saturn (the highest and lowest
density planets in our solar system), yet they are 30 times
closer than any pair of planets in our solar system." (Agol later
clarified to SPACE.com that Kepler-36b and c are actually more
like 20 times closer together than any two planets in our neck of
the woods.) [ Gallery:
The Strangest Alien Planets ]

The two known planets in the Kepler-36 system — which is located
in the constellation Cygnus (The Swan) — were detected by NASA's
Kepler Space Telescope.

Kepler is staring continuously at more than 150,000 stars,
watching for telltale brightness dips caused when planets cross
in front of the stars from the telescope's perspective. Since its
March 2009 launch, Kepler has flagged more than 2,300 potential
alien planets; while only a small fraction have been confirmed to
date, mission scientists think more than 80 percent of them will
end up being the real deal.

Kepler-36c, which is about 3.7 times wider than Earth, likely has
a rocky core surrounded by a substantial atmosphere filled with
lots of hydrogen and helium, researchers said.

Kepler-36b, on the other hand, is a
super-Earth just 1.5 times wider than our planet. Iron likely
constitutes about 30 percent of its mass, water around 15 percent
and atmospheric hydrogen and helium less than 1 percent,
researchers said.

Though they're very different in size and makeup, the two planets
travel on surprisingly similar paths around their host star.
Kepler-36c orbits once every 16 days, at an average distance of
12 million miles (19 million km). Kepler-36b orbits each 14 days
and sits about 11 million miles (18 million km) from the star.

Kepler-36b probably formed relatively close to the star, while
Kepler-36c likely took shape farther out. Astronomers model
large-scale migrations that can bring initially far-flung planets
much closer together, but the peculiar Kepler-36 system may force
some refinements, researchers said.

"These models rely on assumptions that will likely have to be
'tweaked' or refined to account for both b and c's proximity and
compositional differences," Carter told SPACE.com via email. "The
existence of Kepler-36 may help clarify or invalidate these
assumptions."

Both planets are likely too hot to support life as we know it,
with Kepler-36b probably sporting lava flows on its surface. They
orbit roughly three times closer to their host star, known as
Kepler-36a, than the hellishly hot planet
Mercury does to our sun. And Kepler-36a is likely a bit
hotter than our star, researchers said.

The researchers publish their results Thursday (June 21) in the
journal Science.

An impressive sky scene

Every 97 days, Kepler-36b and c experience a conjunction that
brings them within just 1.2 million miles (1.9 million km) of
each other — roughly five times the Earth-moon distance. This
would be quite a sight for an observer on the surface of either
planet.

"Planet c would appear roughly 2.5 times the size of the
full moon when viewed from the surface of planet
b. Conversely, planet b would appear about the size of the
full moon on planet c," Carter said.

"We can speculate on the appearance of planet c: It may appear
slightly more purple that Neptune," he added. "The purple hue
owes to absorption of red and yellow by sodium and
potassium. There could also be a slight brown tint owing to
hazes of photo-disassociated methane."

Such dramatic vistas could well be around for many years to come,
researchers said, for the orbits of Kepler-36b and c appear
unlikely to change anytime soon.

"We are addressing this in a follow-up paper, but the short
answer is that yes, these do appear to be stable on a long
timescale," Agol said.